Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce despite restriction of space, nutrients shared by other cells, or signals sent from the body to stop reproduction. Cancer cells are often shaped differently from healthy cells, do not function properly, and can spread into many areas of the body. Abnormal growths of tissue, called tumors, are clusters of cells that are capable of growing and dividing uncontrollably. Tumors can be benign (noncancerous) or malignant (cancerous). Benign tumors tend to grow slowly and do not spread. Malignant tumors can grow rapidly, invade and destroy nearby normal tissues, and spread throughout the body.
Cancers are classified according to the kind of fluid or tissue from which they originate, or according to the location in the body where they first developed. In addition, some cancers are of mixed types. Cancers can be grouped into five broad categories, carcinomas, sarcomas, lymphomas, leukemias, and myelomas, which indicate the tissue and blood classifications of the cancer. Carcinomas are cancers found in body tissue known as epithelial tissue that covers or lines surfaces of organs, glands, or body structures. For example, a cancer of the lining of the stomach is called a carcinoma. Many carcinomas affect organs or glands that are involved with secretion, such as breasts that produce milk. Carcinomas account for approximately eighty to ninety percent of all cancer cases. Sarcomas are malignant tumors growing from connective tissues, such as cartilage, fat, muscle, tendons, and bones. The most common sarcoma, a tumor on the bone, usually occurs in young adults. Examples of sarcoma include osteosarcoma (bone) and chondrosarcoma (cartilage). Lymphoma refers to a cancer that originates in the nodes or glands of the lymphatic system, whose job it is to produce white blood cells and clean body fluids, or in organs such as the brain and breast. Lymphomas are classified into two categories: Hodgkin's lymphoma and non-Hodgkin's lymphoma. Leukemia, also known as blood cancer, is a cancer of the bone marrow that keeps the marrow from producing normal red and white blood cells and platelets. White blood cells are needed to resist infection. Red blood cells are needed to prevent anemia. Platelets keep the body from easily bruising and bleeding. Examples of leukemia include acute myelogenous leukemia, chronic myelogenous leukemia, acute lymphocytic leukemia, and chronic lymphocytic leukemia. The terms myelogenous and lymphocytic indicate the type of cells that are involved. Finally, myelomas grow in the plasma cells of bone marrow. In some cases, the myeloma cells collect in one bone and form a single tumor, called a plasmacytoma. However, in other cases, the myeloma cells collect in many bones, forming many bone tumors. This is called multiple myeloma.
Tumor induction and progression are often the result of accumulated changes in the tumor-cell genome. Such changes can include inactivation of cell growth inhibiting genes, or tumor suppressor genes, as well as activation of cell growth promoting genes, or oncogenes. Hundreds of activated cellular oncogenes have been identified to date in animal models, however, only a small minority of these genes have proven to be relevant to human cancers (Weinberg et al 1989 Oncogenes and the Molecular Origins of Cancer Cold Spring Harbor, N.Y., Stanbridge and Nowell 1990 Cell 63 867-874, Godwin et al 1992 Oncogenes and antioncogenes in gynecological malignancies. In W J Hoskins, C A Perez and R C Young (eds), Gynecological oncology: principles and practice, pp 87-116, Lippincott, Philadelphia). The activation of oncogenes in human cancers can result from factors such as increased gene copy number or structural changes. These factors can cause numerous cellular effects, for example, they can result in overexpression of a gene product. Several oncogenes involved in human cancer can be activated through gene overexpression.
It has become apparent that the successive genetic aberrations acquired by cancer cells result in defects in regulatory signal transduction circuits that govern normal cell proliferation, differentiation and programmed cell death (Hanahan, D. and R. A. Weinberg, Cell, 2000. 100(1): p. 57-700). This in turn results in fundamental defects in cell physiology which dictate malignancy. These defects include: a) self sufficiency in growth signals (i.e. overexpression of growth factor receptor tyrosine kinases such as EGFR and aberrant activation of downstream signal transduction pathways such as Ras/Raf/Mek/Erk ½ and Ras/PI3K/Akt), b) resistance to anti-growth signals (i.e. lower expression of TGFβ and its receptor), c) evading apoptosis (i.e. loss of proapoptotic p53; overexpression of pro-survival Bcl-2; hyperactivation of survival pathways such as those mediated by PI3K/Akt), d) sustained angiogenesis (i.e. high levels of secretion of VEGF) and f) tissue invasion and metastasis (i.e. extracellular proteases and prometastatic integrins) (Hanahan, D. and R. A. Weinberg, Cell, 2000. 100(1): p. 57-700).
Receptor tyrosine kinases such as EGFR, ErbB2, VEGFR and insulin-like growth factor I receptor (IGF-1R) are intimately involved in the development of many human cancers including colorectal pancreatic, breast and ovarian cancers (Khaleghpour, K., et al. Carcinogenesis, 2004. 25(2): p. 241-8.; Sekharam, M., et al., Cancer Res, 2003. 63(22): p. 7708-16). Binding of ligands such as EGF, VEGF and IGF-1 to their receptors promotes stimulation of the intrinsic tyrosine kinase activity, autophosphorylation of specific tyrosines in the cytoplasmic domain of the receptors and recruitment of signaling proteins that trigger a variety of complex signal transduction pathways (Olayioye, M. A., et al., Embo J, 2000. 19(13): p. 3159-67, Porter, A. C. and R. R. Vaillancourt, Oncogene, 1998. 17(11 Reviews): p. 1343-52). This in turn leads to the activation of many tumor survival and oncogenic pathways such as the Ras/Raf/Mek/Erk ½, JAK/STAT3 and PI3K/Akt pathways. Although all three pathways have been implicated in colon, pancreatic, breast and ovarian oncogenesis, those that are mediated by Akt have been shown to be critical in many steps of malignant transformation including cell proliferation, anti-apoptosis/survival, invasion and metastasis and angiogenesis (Datta, S. R. et al. Genes Dev, 1999. 13(22): p. 2905-27).
Akt is a serine/threonine protein kinase (also known as PKB), which has 3 family members Akt1, Akt2 and Akt3. Stimulation of cells with growth or survival factors results in recruitment to the receptors of the lipid kinase phosphoinositide-3-OH-kinase (PI3K) which phosphorylates phosphoinositol-4,5-biphosphate (PIP2) to PIP3 which recruits Akt to the plasma membrane where it can be activated by phosphorylation on Thr308 and Ser473 (Akt1), Thr308 and Ser474 (Akt2) and Thr308 and Ser472 (Akt3) (Datta, S. R. et al. Genes Dev, 1999. 13(22): p. 2905-27). Thus, PI3K activates Akt by phosphorylating PIP2 and converting to PIP3. The phosphatase PTEN dephosphorylates PIP3 to PIP2 and hence prevents the activation of Akt.
The majority of human cancers contain hyperactivated Akt (Datta, S. R. et al. Genes Dev, 1999. 13(22): p. 2905-27, Bellacosa, A., et al., Int J Cancer, 1995. 64(4): p. 280-5; Sun, M., et al., Am J Pathol, 2001. 159(2): p. 431-7). In particular, Akt is overexpressed and/or hyperactivated in 57%, 32%, 27% and 36% of human colorectal, pancreatic, breast and ovarian cancers, respectivel (Roy, H. K., et al. Carcinogenesis, 2002. 23(1): p. 201-5, Altomare, D. A., et al., J Cell Biochem, 2003. 88(1): p. 470-6., Sun, M., et al., Cancer Res, 2001. 61(16): p. 5985-91., Stal, O., et al. Breast Cancer Res, 2003. 5(2): p. R37-44, Cheng, J. Q., et al., Proc Natl Acad Sci USA, 1992. 89(19): p. 9267-71, Yuan, Z. Q., et al., Oncogene, 2000. 19(19): p. 2324-30). Hyperactivation of Akt is due to amplification and/or overexpression of Akt itself as well as genetic alterations upstream of Akt including overexpression of receptor tyrosine kinases and/or their ligands (Khaleghpour, K., et al. Carcinogenesis, 2004. 25(2): p. 241-8.; Sekharam, M., et al., Cancer Res, 2003. 63(22): p. 7708-16, Cohen, B. D., et al., Biochem Soc Symp, 1998. 63: p. 199-210., Muller, W. J., et al. Biochem Soc Symp, 1998. 63: p. 149-57, Miller, W. E., et al. J Virol, 1995. 69(7): p. 4390-8, Slamon, D. J., et al., Science, 1987. 235(4785): p. 177-82, Andrulis, I. L., et al., J Clin Oncol, 1998. 16(4): p. 1340-9) and deletion of the phosphatase PTEN. Proof-of-concept of the involvement of Akt in oncogenesis has been demonstrated preclinically by showing that ectopic expression of Akt induces malignant transformation and promotes cell survival (Sun, M., et al. Am J Pathol, 2001. 159(2): p. 431-7, Cheng, J. Q., et al., Oncogene, 1997. 14(23): p. 2793-801) and that disruption of Akt pathways inhibits cell growth and induces apoptosis (Jetzt, A., et al. Cancer Res, 2003. 63(20): p. 6697-706).
Current treatments of cancer and related diseases have limited effectiveness and numerous serious unintended side effects. Despite demonstrated clinical efficacy of many anti-cancer drugs, severe systemic toxicity often halts the clinical development of promising chemotherapeutic agents. Further, overexpression of receptor tyrosine kinases such as EGFR and their ligands such as IGF-1 and Akt overexpression and/or loss of PTEN (all of which result in hyperactivation of Akt) are associated with poor prognosis, resistance to chemotherapy and shortened survival time of cancer patients. Current research strategies emphasize the search for effective therapeutic modes with less risk.